Stuff that is relevant (8)
I believe the answer is:
material
I believe this clue is a double definition.
'stuff' is the first definition.
(synonyms)
'relevant' is the second definition.
(similar in meaning)
'that is' acts as a link.
(Other definitions for material that I've seen before include "Tangible substance" , "of serious importance" , "Preoccupied with physical, rather than spiritual, substance" , "Matter; cloth" , "Relevant - fabric" .)